This invention relates generally to methods for the synthesis of carbon-containing compounds and materials such as polycyclic aromatic hydrocarbons, fullerenes, and nanotubes.
Kroto et.al. (Nature, Vol 318(#14), pg162-163, 1985) discovered that laser vaporization of carbon into flowing helium gas results in formation of a new form of carbon, a compound having the composition C60. The new C60 molecule was named xe2x80x9cBuckminsterfullerenexe2x80x9d, or xe2x80x9cBuckyballxe2x80x9d in honor of its almost spheroidal shape, and is a closed truncated icosohedron of carbon atoms, formed from fused five and six-membered aromatic rings.
Subsequent research (see for example Kroto et.al, Nature, Vol 331(#28), pg 328-331, 1988) showed that a much larger family of hollow carbon-cage cluster species generically termed xe2x80x9cfullerenesxe2x80x9d can be generated from carbon-containing substrates. Large closed-cage fullerenes of formulas including C70, C120, C130, C140, C180, and C240 are believed to exist. Other fullerenes with open (i.e. not closed) carbon cages also exist. U.S. Pat. No. 5,876,684 to Withers et.al. is an example of a method and apparatus for producing fullerenes. Other workers have reported formation of fullerenes from vapor phase pyrolysis of napthalene at temperatures of about 1000xc2x0 C. (see Taylor et.al., Nature, Vol 366(#23), pg 728-731,1993).
Research has also shown (see for example Ebbesen et.al., Nature, Vol 358(#16), pg 220-222, 1992) that the family of fullerenes includes hollow graphitic tubules whose dimensions are on the order of nanometers. These hollow graphic tubules have walls formed from sheets of fused six-membered rings. The walls of the nanotubes may comprise many concentric layers of the graphitic sheets, or may have a single layered wall (see for example Satishkumar et.al., Chemical Physics Letters, Vol 293, pg 47-52, 1998).
Fullerene-type ions and molecules have also been spectroscopically detected in the vapor phase of sooting flames (see for example, Gerhardt et.al, Chemical Physics Letters, Vol 137, pg 306-310, 1987; Pope et.al., J. Phys. Chem., Vol 97, pg 1101-1103, 1993). Other workers have extracted fullerenes from soot (see Howard et. al., Nature, Vol 352(#1.1), pg 139-141, 1991;McKinnon et.al., Combustion and Flame, Vol 88, pg 102-112, 1992).
Fullerenes and nanotubes have a variety of uses, including use as superconductors, photo-conductors, micro-lubricants, catalysts, catalyst supports, electrodes for batteries, adsorbents, hydrogen storage media, plant-growth regulators, and pharmaceuticals. In response to such uses, a variety of methods for synthesizing, characterizing, and purifyng fullerenes and nanotubes have been developed (see for example Srivastava, Energy Sources, Vol 17, pg 615-640, 1995). Nevertheless, the yields of fullerenes and nanotubes remain low, and costs of producing and purifying fullerenes remains extremely high, which has significantly limited the commercial viability of many potential applications.
The chemistry of flame combustion, and the process of soot formation in flames has been investigated. For example, it is known that polycyclic aromatic hydrocarbons are formed in flames, and that the polycyclic aromatic hydrocarbons may be precursors of soot in flames (see for example, Dobbins et.al., Combust. Sci.and Tech., Vol. 121, pg 103-121, 1996). Polycyclic aromatic hydrocarbons (xe2x80x9cPAHsxe2x80x9d) are a large class of hydrocarbon compounds having fused five and/or six membered aromatic ring residues. A list of about 622 known polycyclic hydrocarbons has been tabulated by Sanders and Wise of the National Institute of Standards and Technology, in NIST Special Publication 922, available at inter-alia, the NIST website.
Polycyclic hydrocarbons initially and predominately grow in the vapor phase of a flame by step-wise condensation of two carbon fragments. PAH compounds with even numbers of carbon atoms, comprising planar arrays of fused six-membered benzene residues are believed to predominate over PAH compounds with odd numbers of carbon atoms or five-membered rings in flames, because of differences in thermodynamic stability (see Dobbins, et.al., Combustion and Flame, Vol 115, pg 285-298, 1998, and Weilmxc3xcnster et.al., Combustion and Flame, Vol 116, pg 62-83, 1999).
Baum et.al. (Ber Bunsenges Phys. Chem. Vol 96(7), pg. 841-857, 1992) have suggested that fullerenes may form by coagulation or condensation of PAH molecules with other PAH molecules or immature soot particles.
However, there remains a need in the art for improved and lower cost methods for the synthesis of polycyclic aromatic hydrocarbons, fullerenes and nanotubes.
Among other things, the present invention relates to a method for collecting materials from a flame, which materials include unexpectedly large condensed phase, xe2x80x9cliquid-likexe2x80x9d droplets or particles which contain large quantities of polycyclic aromatic hydrocarbons. In addition, the present invention is based on the recognition that PAH containing condensed phases have value, can be collected, and may be practically utilized for purposes other than combustion. In particular, the PAH containing condensed phases may be collected, and subsequently reacted to synthesize other valuable chemical compounds. For example, the condensed phase comprising polycyclic aromatic hydrocarbons can be reacted to produce fullerenes, nanotubes, and other valuable materials.
The inventors have also unexpectedly discovered that fullerenes and nanotubes are advantageously formed in liquid phases containing PAH compounds. While not wishing to be bound by theory, it is believed that the rate of growth of relatively large fullerenes and nanotubes is increased in the liquid phase, by condensation or dimerization reactions between PAH compounds. Because of the high concentrations of PAH compounds which may be produced in liquid phases, high reaction rates can be obtained.
Therefore, this invention, in one aspect, relates to a method for producing a polycyclic aromatic hydrocarbon comprising:
a. condensing at least one carbon-containing material in a flame to form a condensed phase; and
b. collecting at least a portion of the condensed phase from the flame; wherein the condensed phase comprises at least one polycyclic aromatic hydrocarbon.
In another aspect, the invention relates to a method for producing fullerenes and nanotubes comprising:
a. heating at least one carbon-containing material to form a condensed phase comprising at least one polycyclic aromatic hydrocarbon;
b. collecting at least some of the condensed phase;
c. reacting the condensed phase to form fullerenes and/or nanotubes.
In other aspects, the invention relates to the products produced by the processes of the invention.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.